Integrated circuits are typically formed on substrates, particularly silicon wafers, by the sequential deposition of conductive, semi-conductive or insulative layers. After each layer is deposited, the layer is etched to create circuitry features. As a series of layers are sequentially deposited and etched, the outer or uppermost surface of the substrate, i.e., the exposed surface of the substrate, becomes successively more non-planar. This occurs because the distance between the outer surface and the underlying substrate is greatest in regions of a substrate where the least etching has occurred, and least in regions where the greatest etching has occurred. For a single-patterned underlying layer, this non-planar surface comprises a series of peaks and valleys wherein the distance between the highest peak and the lowest valley may be on the order of 7,000 to 10,000 Angstroms. With multiple-patterned underlying layers, the height difference between the peaks and valleys becomes even more severe, and can reach several microns.
This non-planar outer surface presents a problem for the integrated circuit manufacturer. If the outer surface is non-planar, then photolithographic techniques to pattern photoresist layers might not be suitable, as a non-planar surface can prevent proper focusing of the photolithography apparatus. Therefore, there is a need to periodically planarize this substrate surface to provide a planar layer surface. Planarization, in effect, polishes away a non-planar outer surface, whether a conductive, semi-conductive, or insulative layer, to form a relatively flat, smooth surface. Following planarization, additional layers may be deposited on the outer layer to form interconnect lines between features, or the outer layer may be etched to form vias to lower features.
Chemical-mechanical polishing is one accepted method of planarization. This planarization method typically requires that the substrate be mounted on a carrier or a polishing head, with the surface of the substrate to be polished exposed. The substrate is then placed against a rotating polishing pad. In addition, the carrier head may rotate to provide additional motion between the substrate and polishing surface. Further, a polishing slurry, including an abrasive and at least one chemically-reactive agent, may be spread on the polishing pad to provide an abrasive chemical solution at the interface between the pad and substrate.
Important factors in the chemical-mechanical polishing process are: the finish (roughness) and flatness (lack of large-scale topography) of the substrate surface, and the polishing rate. Inadequate flatness and finish can produce substrate defects. Polishing rate sets a time needed to polish a layer. Thus, it sets the maximum throughput of the polishing apparatus.
Each polishing pad provides a surface, which, in combination with the specific slurry mixture, can provide specific polishing characteristics. Thus, for any material being polished, the pad and slurry combination is theoretically capable of providing a specified finish and flatness on the polished surface. The pad and slurry combination can provide this finish and flatness in a specified polishing time. Additional factors, such as the relative speed between the substrate and pad, and the force pressing the substrate against the pad, affect the polishing rate, finish and flatness.
Because inadequate flatness and finish can create defective substrates, the selection of a polishing pad and slurry combination is usually dictated by the required finish and flatness. Given these constraints, the polishing time needed to achieve the required finish and flatness sets the maximum throughput of the polishing apparatus.
An additional limitation on polishing throughput is "glazing" of the polishing pad. Glazing occurs when the polishing pad is heated and compressed in regions where the substrate is pressed against it. The peaks of the polishing pad are pressed down and the pits of the polishing pad are filled up, so the surface of the polishing pad becomes smoother and less abrasive. As a result, the polishing time required to polish a substrate increases. Therefore, the polishing pad surface must be periodically returned to an abrasive condition, or "conditioned" to maintain a high throughput.
An additional consideration in the production of integrated circuits is process and product stability. To achieve a low defect rate, each successive substrate should be polished under similar conditions. Each substrate should be polished by approximately the same amount so that each integrated circuit is substantially identical.
One of the factors that is accounted for in returning the polishing pad to its condition prior to the polishing of the wafer is the removal from the polishing pad of the debris created during the polishing period. These debris may be on the surface of the polishing pad or trapped within grooves of the polishing pad. If the debris are left on and within the pad, the polishing conditions for the next pad to be polished will be different from the previous pad that was just polished.
A typical method of removing the debris from the polishing pad after a wafer has been polished is to employ a spray rinse arm over the surface of the polishing pad. The spray rinse arm provides a rinse of de-ionized water to wash away the debris from the polishing pad once a wafer has been polished. Although the use of a spray rinse arm provides some measure of cleaning to the polishing pad, some debris may remain behind, especially within grooves in the polishing pad. One possible way to increase the cleaning action of the spray rinse arm is to employ a brush that would mechanically loosen the debris from the polishing pad. Such brush heads are not typically used, however, since bristles from conventional brushes are not adequately secured to prevent their dislodgement from the brush onto the polishing pad surface. Hence, even after cleaning of the polishing pad, a brush bristle may be present on a polishing pad. A bristle that remains on a polishing pad may scratch a wafer that is subsequently polished on the pad.
One of the methods to secure bristles together in typical brushes is to staple the bristles together and then force the stapled bristles into brush holes, where the bristles expand within the brush holes. However, this method of attaching the bristles to the brush does not adequately secure the bristles into the brush and they typically fall out of the brush.
A second method to attach bristles to a brush is to hand-tie the bristles into bundles, push the bundles through a brush head and tie the bristle bundles together. However, in order to hand-tie the bristles, a very large number of bristles is needed in each bundle, and the bristle bundles become much too stiff for use in a pad cleaning brush.
A third possible method of securing bristles to a brush is to epoxy the end of the bristles to the brush. However, this is problematic, since the viscosity of the epoxy used to secure the bristles makes it difficult, if not impossible, to force the bristles into the epoxy.